A polygutamine expansion in the N-terminus of huntingtin (N-hn) causes Huntington's disease (HD). There is no effective treatment for HD. Although mutant N-Fin fragments are known to accumulate in HD neurons and cause cell dysfunction in vitro, a mechanism (or mechanisms) that explains the selective loss of striatal and cortical projection neurons remains elusive. Wild type and mutant htt associate with membranes in the endocytic and secretory pathway. Our overall hypothesis is that mutant N-htt 's association with neuronal membranes contributes to early cellular dysfunction in the cytoplasm in HD. One of these membrane compartments includes the autophagosome/lysosomal system, which accumulates full-length or large N-htt fragments of mutant htt. Although htt ostensibly lacks transmembrane domains, it associates tightly with membranes. We speculate that candidate domains in the N-terminus involved in protein-protein interactions promote membrane binding. Little is known about the degradative pathways that form N-htt fragments in vivo. Identifying the sites of protease cleavage in the N-terminus of htt is important for understanding how the protein is regulated. Calpain, a calcium dependent protease, which regulates the function of many proteins involved in membrane/cytoskeleton organization, cleaves htt near its N-terminus and produces long-lived N-htt fragments that are enriched in membrane fractions in brain. We speculate that mutant N-htt products of calpain cleavage undergo a different processing from the wt fragments in neurons that leads to cellular dysfunction in HD. HD mice also show abnormal function of striatal NMDA receptors. One way that mutant Fin might cause the dysfunction of NMDA receptors is by disrupting the assembly of NMDA receptor subtypes at the cell surface. The specific aims are: 1: To understand the role of mutant htt induced autophagy in cell dysfunction, 2: To determine whether htt proteolysis by calpain contributes to HD pathogenesis and 3: To determine the effects of mutant htt on protein transport within the secretory pathway. The results of these studies will provide new insights about the mechanisms of cellular dysfunction in HD and suggest novel therapeutic targets that can reduce the potentially harmful effects of mutant N-htt fragments.